A host resistance gene has been identified in mice which specifically suppresses in vitro and in vivo replication of the MCF class of murine leukemia viruses. Designated R-Mcf, the gene is dominant for resistance, is located on chromosome 5 very close to the locus for hammer-toe (Hm), and is carried by DBA/1, DBA/2 and certain sublines of CBA (i.e., those related to CBA/H). In F1 hybrids between AKR females and males of various Fv-1n strains, the presence of the R-McfR allete results in significant delays in onset of spontaneous thymic lymphoma and in much reduced amplication of MuLV antigens and MCF virus in thymus. There is a similar but less dramatic effect on MCF virus accelerated disease in the F1's. The effect of this gene on other spontaneous and virus induced neoplasia is under study, along with other host genes which affect virus replication and spread.